This invention relates to a process for friction-welding steel to cast iron that contains nodular graphite, comprising the steps of (a) generating a relative movement, by rotation, between contacting surfaces of parts to be joined, (b) pressing the parts to be joined against each other along their contacting surfaces during the generation of relative movement (i.e., at a frictional pressure), and (c) pressing the parts to be joined during a subsequent stationary period at an elevated pressure (i.e., at a forging pressure).
According to DVS (German Welding Engineering Association) instruction sheet 2909 part 1 (March 1980), a distinction is made between friction-welding with continuous drive, friction-welding with inertia drive and a combined friction-welding (hybrid friction-welding). In the case of the last variant, rotary speed, frictional force per unit area, frictional time/frictional path, braking instant, forging instant, forging force per unit area and forging time emerge as main welding parameters.
In essays by Professor Dr. H. Richter and Dipl.-Ing. A. Palzkill in Schweissen und Schneiden 37: 60-65 (1985) ("Ubertragung von Mikroreibschweissversuchen auf bauteilgrosse Proben am beispiel der Werkstoffkombination Baustahl/Gausseisen mit Kugelgraphit") ["Extrapolation of micro-friction welding experiments to component-size samples based on the example of the material combination comprising structural steel/cast iron containing nodular graphite"] and in Konstruieren und Giessen 11: 33-37 (1986) ("Friction welding of steel to cast iron containing nodular graphite"), a friction-welding process is described, substantially as below:
The process of friction-welding is based on heat development generated by friction between contacting surfaces of work-pieces to be joined. For this purpose, in addition to application of a relative rotary movement to generate friction, pressure is applied. Within the context of the present invention, the step at which friction is generated is otherwise referred to as a frictional process, the period during which friction is generated is otherwise referred to as frictional time, and the pressure applied during the frictional process is otherwise referred to as frictional pressure. The frictional process proceeds within a time period of fractions of a second to a few seconds so that heat generated at a point of contact results in temperatures of up to approximately the melting point of one of the workpieces. The contacting surfaces become highly plastic during this process and are, in general, displaced out of the welding zone under the action of frictional force so that a flash is formed. In the welding zone itself, atomic structures of the contacting surfaces are brought into close contact during this frictional process such that elementary binding forces become effective. The correct combined action of rotary speed or frictional velocity, frictional force, possibly forging force at the end of the frictional process, and also the frictional time are decisive for success of a welded joint. In friction-welding combination of steel and graphite-containing cast iron, action of graphite from the cast iron on the weld must also be taken into account, because this graphite builds up a lubricating layer, which layer impedes generation of an intensive frictional force and, consequently, development of heat.
Based upon these publications, it appears that a bearing joint of steel and cast iron that contains nodular graphite can be made by friction welding, but this process would result in a weld interface structure and a weld point that are characterized by a change in graphite formation and in basic structure of the joint. Although such joints exhibit metallurgically satisfactory bonding zones, they do not adequately resist mechanical loads.